The LSI for mobile communication and portable information appliances is advanced in digitization of circuitry mainly in the signal processing signal and control circuit. A mixed analog/digital LSI including analog signal processing circuitry for input signals is a key device for realizing small size and low power consumption of appliances. For such high performance, analog circuit design technology of high precision is needed.
For analog circuit design, it is important to understand the precision of devices formed on a semiconductor substrate (transistor, resistor, capacitor, etc.).
On the other hand, in the devices on the semiconductor substrate, there are always manufacturing fluctuations having a specific tendency. As the approach on the LSI manufacturing process side, process parameters for reducing as much as possible such fluctuations due to a manufacturing device are employed. Yet, fluctuations of created devices are not eliminated to zero.
The circuit designer must select circuit parameters having a margin for guaranteeing normal operation, even in a worst case, while understanding the relative precision (parameter precision of two adjacent devices) of the device parameters due to such manufacturing fluctuations (capacitance, resistance, etc.). If the operation margin is too large, the circuit performance is sacrificed. Such being the case, a designer's understanding of accurate device fluctuations leads to higher performance of LSI.
A prior art understanding of such device fluctuations is described by reference to FIG. 9 and FIG. 10 which show an example of capacitors, in particular, as passive devices. FIG. 9 and FIG. 10 show conventional patterns for evaluating the capacitance and capacitor specific precision. In FIG. 9, reference numerals 8 and 9 are unit capacitors having capacity of 50 fF each, 10 and 11 are upper electrode terminals, and 12 is a common GND terminal. In FIG. 10, reference numerals 13 and 14 are unit capacitors having capacity of 10 pF each, 15 and 16 are upper electrode terminals, and 17 is a common GND terminal.
In the conventional evaluation method of capacitor specific precision, as shown in FIG. 9, two independent devices with small capacitance used in an actual circuit (for example, 50 fF: supposing capacity density to be 1 fF/.mu.m.sup.2, area in 50 .mu.m.sup.2 : approx. 7 .mu.m square) are disposed closely to each other, the capacitances are measured, and their difference is calculated to define a specific precision, or, as shown in FIG. 10, such devices of large capacitances as the measuring errors may be ignored (about 10 pF: supposing capacity density to be 1 fF/.mu.m.sup.2, area is 10000 .mu.m.sup.2 : 100 .mu.m square) are disposed closely to each other, and the difference of the capacitances is evaluated, and the specific precision is defined.
These conventional evaluation methods, however, have the following problems.
First, in an actual circuit, capacitors of small individual capacity such as 30 fF or 50 fF are used, and what is required in circuit design is the difference of such proximity capacitances, but in the present capacity measuring instrument of high precision, it is extremely hard to measure the capacitance or the order of even 10 fF at high precision, and the obtained value is concealed by the measuring error, and the meaning of specific precision is practically lost.
Second, the capacitor of about 10 pF which is considered to be measured precisely enough by the existing capacity measuring instrument has an area of 10000 .mu.m.sup.2, for example, supposing the capacity density to be 1 fF/.mu.m.sup.2, that is 100 .mu.m square. By disposing such unit capacities close to each other, when the difference of the capacitances is determined, a sufficient measuring precision is obtained, while the capacitor specific precision as measured above is not suitable for actual circuit design.
The problems of capacitors are mentioned above, but same problems exist in the evaluation of resistors.